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Late March Astronomy Bulletin

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ASTRONOMERS FIND BINARY BROWN DWARF
University of Birmingham

Astronomers working on 'first light' results from a newly commissioned telescope in Chile made a chance discovery that led to the identification of a rare eclipsing binary brown dwarf system. The discovery was made by an international team of researchers working on the SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) project. SPECULOOS' mission is to investigate planets surrounding ultra-cool dwarfs, a category that includes the smallest stars that exist, as well as objects called 'brown dwarfs'. Brown dwarfs are 'sub-stellar' objects, meaning they have less mass than a star but more than a planet. Brown dwarfs are unable to sustain the fusion of hydrogen into helium, a process that powers the light from normal stars like the Sun. Astronomers predict that these ultra-cool dwarfs should host large populations of close-by, potentially habitable rocky planets, offering a wealth of opportunity to explore a diversity of atmospheres and climates.  An example is the 7-planet system TRAPPIST-1, which was discovered by members of the same team. Soon after the construction the first SPECULOOS telescopes, and during testing observations, the team targeted the known brown dwarf 2MASSW J1510478-281817, since renamed 2M1510, in the constellation Libra. The SPECULOOS observations picked up a distinct signal that led the researchers to speculate that 2M1510 might be two brown dwarfs instead of one, in orbit around each other.

The team turned one of the telescopes to a known brown dwarf. But suddenly the object appeared to get dimmer for about 90 minutes, which indicated an eclipse just took place. Astronomers realised that they were probably looking at two eclipsing brown dwarfs, one passing in front of the other, a configuration which is much rarer than planetary systems. The researchers were able to confirm their hypothesis using two more powerful telescopes, the 10m Keck Telescope in Hawaii, and the 8m Very Large Telescope in Chile. The VLT is based at the same site as the SPECULOOS telescopes used to make the observations. Keck and VLT have sensitive spectrometers that can be used to measure the velocities of celestial objects. In the case of 2M1510, the astronomers detected the velocities of both brown dwarfs as they orbit one another. The detection of eclipsing brown dwarfs is extremely rare -- only one other such system has been identified to date. These systems provide astronomers the opportunity to measure the brown dwarfs' radii and masses directly, which are fundamental quantities for theoretical models.   2M1510 is also special in that it is among the very few brown dwarfs that has a known age, due to its membership in a nearby cluster of young stars called the Argus moving group.


DIMMING BETELGEUSE JUST DUSTY
University of Washington

Late last year, news broke that the star Betelgeuse was fading significantly, ultimately dropping to around 40% of its usual brightness. The activity fuelled popular speculation that the red supergiant would soon explode as a massive supernova. But astronomers have more benign theories to explain the star's dimming behaviour. And scientists believe they have support for one of them: Betelgeuse isn't dimming because it's about to explode—it's just dusty.  Observations of Betelgeuse taken Feb. 14 at the Flagstaff, Arizona, observatory allowed them to calculate the average surface temperature of the star. They discovered that Betelgeuse is significantly warmer than expected if the recent dimming were caused by a cooling of the star's surface. The new calculations lend support to the theory that Betelgeuse—as many red supergiant stars are prone to do—has likely sloughed off some material from its outer layers. Astronomers see this all the time in red supergiants, and it's a normal part of their life cycle. Red supergiants will occasionally shed material from their surfaces, which will condense around the star as dust. As it cools and dissipates, the dust grains will absorb some of the light heading toward us and block our view. It is still true: Astronomers expect Betelgeuse to explode as a supernova within the next 100,000 years when its core collapses. But the star's dimming, which began in October, wasn't necessarily a sign of an imminent supernova.

Another theory is that huge convection cells within Betelgeuse had drawn hot material up to its surface, where it had cooled before falling back into the interior. A simple way to tell between these possibilities is to determine the effective surface temperature of Betelgeuse. Measuring a star's temperature is no straightforward task. Scientists can't just point a thermometer at a star and get a reading. But by
looking at the spectrum of light emanating from a star, astronomers can calculate its temperature. The light from bright stars is often too strong for a detailed spectrum, but astronomers employed a filter that effectively "dampened" the signal so they could mine the spectrum for a particular signature: the absorbance of light by molecules of titanium oxide. Titanium oxide can form and accumulate in the upper layers of large, relatively cool stars like Betelgeuse. It absorbs certain wavelengths of light, leaving telltale "scoops" in the spectrum of red supergiants that scientists can use to determine the star's surface temperature. By their calculations, Betelgeuse's average surface temperature on Feb. 14 was about 3,325 degrees Celsius, That's only 50-100 degrees Celsius cooler than the temperature that a team had calculated as Betelgeuse's surface temperature in 2004, years before its dramatic dimming began. These findings cast doubt that Betelgeuse is dimming because one of the star's massive convection cells had brought hot gas from the interior to the surface, where it had cooled. Many stars have these convection cells, including our own sun. They resemble the surface of a pot of boiling water. But whereas the convection cells on our Sun are numerous and relatively small —red
supergiants like Betelgeuse, which are larger, cooler and have weaker gravity, sport just three or four massive convection cells that stretch over much of their surfaces.  If one of these massive cells had risen to Betelgeuse's surface, astronomers would have registered a substantially greater decrease in temperature than what they see between 2004 and 2020. A comparison with our 2004 spectrum showed immediately that the temperature hadn't changed significantly which showed the answer had to be dust.


EXOPLANET RAINS IRON
ESO

Researchers using the Very Large Telescope (VLT) have observed an extreme planet where they suspect it rains iron. The ultra-hot giant exoplanet has a day side where temperatures climb above 2400 degrees Celsius, high enough to vaporise metals. Strong winds carry iron vapour to the cooler night side where it condenses into iron droplets. Known as WASP-76b, it is located some 640 light-years away in the constellation of Pisces. This strange phenomenon happens because the 'iron
rain' planet only ever shows one face, its day side, to its parent star, its cooler night side remaining in perpetual darkness. Like the Moon on its orbit around the Earth, WASP-76b is ‘tidally locked’: it takes as long to rotate around its axis as it does to go around the star. On its day side, it receives thousands of times more radiation from its parent star than the Earth does from the Sun. It’s so hot that molecules separate into atoms, and metals like iron evaporate into the atmosphere. The extreme temperature difference between the day and night sides results in vigorous winds that bring the iron vapour from the ultra-hot day side to the cooler night side, where temperatures decrease to around 1500 degrees Celsius. Not only does WASP-76b have different day-night temperatures, it also has distinct day-night chemistry, according to the new study. Using the new ESPRESSO instrument on  ESO’s VLT in the Chilean Atacama Desert, the astronomers identified for the first time chemical variations on an ultra-hot gas giant planet. They detected a strong signature of iron vapour at the evening border that separates the planet’s day side from its night side.


NEW TYPE OF PULSATING STAR DISCOVERED
University of Sydney

A star that pulsates on just one side has been discovered in the Milky Way about 1500 light years from Earth. It is the first of its kind to be found and scientists expect to find many more similar systems as technology to listen inside the beating hearts of stars improves. The star is known as HD74423, which is about 1.7 times the mass of the Sun. Stars that pulsate have been known in astronomy for a long time. Our own Sun dances to its own rhythms. These rhythmic pulsations of the stellar surface occur in young and in old stars, and can have long or short periods, a wide range of strengths and different causes. There is however one thing that all these stars had thus far in common: the oscillations were always visible on all sides of the star. Now an international team has discovered a star that oscillates largely over one hemisphere. The scientists have identified the cause of the unusual single -sided pulsation: the star is located in a binary star system with a red dwarf. Its close companion distorts the oscillations with its gravitational pull. The clue that led to its discovery came from citizen scientists poring over public data from NASA's TESS satellite, which is hunting for planets around distant stars. The orbital period of the binary system, at less than two days, is so short that the larger star is being distorted into a tear-drop shape by the gravitational pull of the companion. To their surprise the team observed that the strength of the pulsations depended on the aspect angle under which the star was observed, and the corresponding orientation of the star within the binary. This means the pulsation strength varies with the same period as that of the binary. This is how the astronomers could be certain that the pulsations were only found on one side of the star, with the tiny fluctuations in brightness always appearing in their observations when the same hemisphere of the star was pointed towards the telescope. The discovery of the unusual behaviour of the star was initially made by citizen scientists. These amateur astronomy sleuths painstakingly inspected the enormous amounts of data that TESS regularly supplies, as they search for new and interesting phenomena. While this is the first such star to be found where only one side is pulsating, the authors believe there must be many more such stars.


DEEP SPACE NETWORK’S ANTENNA UPGRADE
NASA

Since early March, Voyager 2 has been quietly coasting through interstellar space without receiving commands from Earth. That's because the Voyager's primary means of communication, the Deep Space Network's 70-metre-wide radio antenna in Canberra, Australia, will be undergoing critical upgrades for about 11 months.  During this time, the Voyager team will still be able to receive science data from Voyager 2 on its mission to explore the outermost edge of the Sun's domain and beyond. About the size of a 20-story office building, the dish has been in service for 48 years. Some parts of the 70-metre antenna, including the transmitters that send commands to various spacecraft, are 40 years old and increasingly unreliable.  The Deep Space Network (DSN) upgrades are planned to start now that Voyager 2 has returned to normal operations, after accidentally overdrawing its power supply
and automatically turning off its science instruments in January. The network operates 24 hours a day, 365 days a year and is spread over three sites around the world, in California, Spain and Australia. This allows navigators to communicate with spacecraft at the Moon and beyond at all times during Earth's rotation. Voyager 2, which launched in 1977, is currently more than 17 billion kilometres from Earth. It is flying in a downward direction relative to Earth's orbital plane, where it can be seen only from the southern hemisphere and thus can communicate only with the Australian site. The repairs will benefit far more than Voyager 2, including future missions like the Mars 2020 rover and Moon to Mars exploration efforts. The network will play a critical role in ensuring communication and navigation support for both the precursor Moon and Mars missions and the crewed Artemis missions.


IMPACT OF SATELLITE CONSTELLATIONS
ESO

Astronomers have recently raised concerns about the impact of satellite mega-constellations on scientific research. To better understand the effect these constellations could have on astronomical observations, ESO commissioned a scientific study of their impact, focusing on observations with ESO telescopes in the visible and infrared but also considering other observatories. The study, considers a total of 18 representative satellite constellations under development by SpaceX, Amazon, OneWeb and others, together amounting to over 26 thousand satellites. It finds that large telescopes like ESO's Very Large Telescope (VLT) and ESO's upcoming Extremely Large Telescope (ELT) will be "moderately affected" by the constellations under development. The effect is more pronounced for long exposures (of about 1000 s), up to 3% of which could be ruined during twilight, the time between dawn and sunrise and between sunset and dusk. Shorter exposures would be less impacted, with fewer than 0.5% of observations of this type affected. Observations conducted at other times during the night would also be less affected, as the satellites would be in the shadow of the Earth and therefore not illuminated.  Depending on the science case, the impacts could be lessened by making changes to the operating schedules of ESO telescopes, though these changes come at a cost. On the industry side, an effective step to mitigate impacts would be to darken the satellites. The study also finds that the greatest impact could be on wide-field surveys, in particular those done with large telescopes. For example, up to 30% to 50% of exposures with the US National Science Foundation's Vera C. Rubin Observatory would be "severely affected”, depending on the time of year, the time of night, and the simplifying assumptions of the study. Mitigation techniques that could be applied on ESO telescopes would not work for this observatory although other strategies are being actively explored. Further studies are required to fully
understand the scientific implications of this loss of observational data and complexities in their analysis.

Wide-field survey telescopes like the Rubin Observatory can scan large parts of the sky quickly, making them crucial to spot short-lived phenomena like supernovae or potentially dangerous asteroids. Because of their unique capability to generate very large data sets and to find observation targets for many other observatories, astronomy communities and funding agencies in Europe and elsewhere have ranked wide-field survey telescopes as a top priority for future developments in astronomy. Professional and amateur astronomers alike have also raised concerns about how satellite mega-constellations could impact the pristine views of the night sky. The study shows that about 1600 satellites from the constellations will be above the horizon of an observatory at mid-latitude, most of which will be low in the sky — within 30 degrees of the horizon. Above this — the part of the sky where most astronomical observations take place — there will be about 250 constellation satellites at any given time. While they are all illuminated by the Sun at sunset and sunrise, more and more get into the shadow of the Earth toward the middle of the night. The ESO study assumes a brightness for all of these satellites. With this assumption, up to about 100 satellites could be bright enough to be visible with the naked eye during twilight hours, about 10 of which would be higher than 30 degrees of elevation. All these numbers plummet as the night gets darker and the satellites fall into the shadow of the Earth. Overall, these new satellite constellations would about double the number of satellites visible in the night sky to the naked eye above 30 degrees. These numbers do not include the trains of satellites visible immediately after launch. Whilst spectacular and bright, they are short lived and visible only briefly after sunset or before sunrise, and — at any given time — only from a very limited area on Earth. Satellite constellations will also have an impact on radio, millimetre and submillimetre observatories, including the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX). This impact will be considered in further studies.


SCIENTISTS SHED LIGHT ON DARK MATTER
University of York

Up to 80% of the Universe could be dark matter, but despite many decades of study, its physical origin has remained an enigma. While it cannot be seen directly, scientists know it exists because of its interaction via gravity with visible matter like stars and planets. Dark matter is composed of particles that do not absorb, reflect or emit light. Now, nuclear physicists are putting forward a new candidate for the mysterious matter -- a particle they recently discovered called the d-star hexaquark.  The particle is composed of six quarks -- the fundamental particles that usually combine in trios to make up protons and neutrons. Importantly, the six quarks in a d-star result in a boson particle, which means that when many d-stars are present they can combine together in very different ways to the protons and neutrons. The research group suggest that in the conditions shortly after the Big Bang, many d-star hexaquarks could have grouped together as the Universe cooled and expanded to form the fifth state of matter -- Bose-Einstein condensate. The next step to establish this new dark matter candidate will be to obtain a better understanding of how the d-stars interact -- when do they attract and when do they repel each other.

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